Diode system having zener diodes and a generator

ABSTRACT

A diode system and a generator, uses Zener diodes. The diode system has AC voltage terminals and DC voltage terminals, in which the Zener diodes are operable in the forward direction to rectify an AC voltage present at the AC voltage terminals, in which a lower limit is provided for the Zener voltage of the Zener diodes, and in which the lower limit of the Zener voltage is provided so that it is lower than a preset lower DC voltage.

FIELD OF INVENTION

The present invention relates to a diode system having Zener diodes anda generator.

BACKGROUND INFORMATION

Three-phase claw-pole generators having a passive rectifier bridge maybe used to generate power in 14 V vehicle electrical systems. In thecase of a load dump (a sudden drop in load), which is produced forexample by quickly cutting off a high load current, a high free-runningvoltage occurs at the generator output, due to a delayed response on thepart of the generator output voltage regulator. To avoid overvoltagedamage in vehicle electrical systems, this overvoltage must be limited.Zener diodes may be used for this purpose in general rectifiers, whichmay be an economical approach to this problem. The rectifier diodes,i.e., Zener diodes, are used during normal operation to rectify thealternating phase voltage produced by the generator. In this case, theZener diodes are operated in the forward direction and have a currentflowing from the anode to the cathode.

In 14 V vehicle electrical systems, the rectifier Zener diodes meetcertain tolerance conditions, the maximum voltage tolerated by theelectrical system from the generator, in extreme cases, being double thenormal electrical system voltage or even higher. These ratios may not beeasily transferable to vehicle electrical systems having higher supplyvoltages because doubling this high supply voltage would result in sucha high voltage that further protective measures would be required.

SUMMARY OF THE INVENTION

The diode system according to the present invention and the generatoraccording to the present invention provide an overload protection forthe vehicle electrical system even in the event of an elevated supplyvoltage for a DC system, economically obtainable Zener diodes being usedas the rectifier diodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a generator having a rectifier and a connected DC system.

FIG. 2 shows a rectifier system according to the present invention.

FIG. 3 shows an example of the tolerance range according to an exemplaryembodiment of the present invention for rectifier diodes according tothe present invention.

DETAILED DESCRIPTION

FIG. 1 shows a generator 20 having a rectifier system 10. It alsoillustrates a controller 30 for generator 20, which is connected togenerator 20. Rectifier system 10 is connected to generator 20 so thatthe outputs of generator 20 (not illustrated in FIG. 1) carrying the ACvoltage supplied by generator 20 correspond to the AC voltage terminals(also not illustrated in FIG. 1) of rectifier system 10. Rectifiersystem 10 includes two DC voltage terminals, a first DC voltage terminalB+ and a second DC voltage terminal B−, which are connected to a DCsystem 100. Rectifier system 10 may additionally include capacitors forsmoothing the rectified voltage at DC voltage outputs B+ and B−,capacitors of this type, however, not being illustrated. DC system 100includes a connection 90 between first DC voltage terminal B+ and DCsystem 100. Connection 90 establishes a connection to the rest of DCsystem 100. DC system 100 also includes a battery 60 and usually a firstload 40 and a second load 50, which may be connected to DC system 100 aswell as disconnected therefrom, activated and deactivated by switchesthat are illustrated, but not identified in further detail.

According to the exemplary embodiment of the present invention, DCsystem 100 is provided, in particular, in the form of a vehicleelectrical system. As a result, the terms vehicle electrical system 100and DC system 100 are used interchangeably below.

FIG. 2 shows an enlarged representation of a rectifier system 10according to FIG. 1. Rectifier system 10 includes a number of diodes,which is why rectifier system 10 according to the present invention isalso referred to as diode system 10. Diode system 10 includes a firstdiode 2, a second diode 3, a third diode 4, a fourth diode 5, a fifthdiode 6, and a sixth diode 7. Diodes 2–7 are provided according to thepresent invention in the form of Zener diodes and form a rectifierbridge of a conventional design. FIG. 2 shows AC terminals on the leftside of diode system 10. The latter include a first AC voltage terminalU, a second AC voltage terminal V, and a third AC voltage terminal W. Onthe right side of rectifier system 10, FIG. 2 shows DC terminals B+, B−of diode system 10.

During normal operation, i.e., when rectifying the AC phase voltagepresent at the AC voltage inputs, i.e., AC voltage terminals U, V, W,diodes 2–7 are operated in the forward direction and have a currentflowing from the anode to the cathode. This is shown in FIG. 2 by brokenline 1 made up of short dashes. When rectifying a positive half-wave ofthe AC voltage between AC voltage terminals U and V, the current pathfirst leads from first AC voltage terminal U to first diode 2, which isactivated in the forward direction; then it leads via first DC terminalB− and vehicle electrical system 100 to second DC terminal B−, fromwhere it passes on to second AC voltage terminal V via fifth diode 6.Both first diode 2 and fifth diode 6 are operated in the forwarddirection.

If a load dump occurs, the AC phase voltage increases and is limited forthe diode combination situated between the phase terminals, i.e., ACterminals U, V, W. Diodes 2–7 are operated at least partially in theirZener breakdown, i.e., in the reverse direction. In this case, thelimiting voltage is determined primarily by the Zener voltage of diodes2–7 that are operated in the reverse direction. The phase limitingvoltage is derived from the sum of a diode forward voltage and a diodeZener voltage. A load dump situation of this type uses a current paththat is illustrated in FIG. 2 by broken current path line 13 made up oflonger dashes. Current path 13 leads from first AC voltage terminal Uvia first diode 2 (in the forward direction) and second diode 3(operated in the reverse direction) to second AC voltage terminal V aswell as via fourth diode 5 (operated in the reverse direction) and fifthdiode 6 (operated in the forward direction), also to second AC voltageterminal V. In this case, the vehicle system limiting voltage derivedfrom the difference between the diode Zener voltage and the diodeforward voltage is present at DC voltage terminals B+, B−. Because thediode Zener voltage is much higher than the diode forward voltage, theload dump limiting voltage present at DC terminals B+, B− is ultimatelydetermined primarily by the breakdown voltage, i.e., the Zener voltageof Zener diodes 2–7. This, in turn, results from the manufacturingtolerance of the Zener voltage of Zener diodes 2–7, which is specifiedunder standardized conditions at low current and room temperature, andalso from other components that are determined by the Zener diodeoperating situation, for example from the reverse current, the junctiontemperature of the diode and similar factors. On the whole, this yieldsa tolerance zone for the load dump limiting voltage, which is determinedby the manufacturing tolerances and operating conditions (currents,junction temperatures) of the diodes. The junction temperature, in turn,is dependent on the external operating temperature, the internal powerloss and the thermal cooling of the diodes in each application. In orderto use the rectifier Zener diodes to effectively limit the load dumpvoltage, they must be designed to ensure that their Zener voltage doesnot exceed or drop below preset tolerance ranges in all operating areasdetermined by the application or by other requirements such as standardsand safety regulations.

According to the exemplary embodiment of the present invention,rectifier Zener diodes 2–7 may also be used for vehicle electricalsystems 100 that have a much higher setpoint voltage than, for example,the 14 V setpoint voltage of conventional vehicle electrical systems100. To achieve this, the breakdown voltage tolerances must be lowered.Under extreme conditions, however, the Zener diode is temporarilyoperated in the Zener breakdown even during normal operation (i.e.,without load dumping) of generator 20 and rectifier 10, while—accordingto the exemplary embodiment of the present invention—the maximum loaddump voltage at generator output B+, B− may rise to a value that ishigher than the permissible voltage in entire vehicle electrical system100 in the event of a complete load dump, for example, the interruptionof connection 90. In this case, all generator components must beconfigured or selected for this elevated voltage, while this voltage isnot applied to any other systems, such as loads 40, 50 and battery 60,because they are disconnected due to the reduction in the generator(power output). A load dump in which at least one further system remainsconnected in addition to the generator electronics results in only aminor load on the diodes due to the load dump, which is accounted for asreduced load dump when the diodes are designed for maintaining the uppervoltage limit in vehicle electrical system 100, i.e., a load dump isconsidered whose magnitude is less than that of a complete load dump. Inthis case, the DC voltage at B+ and B− does not exceed the maximumpermissible voltage in vehicle electrical system 100 according to thepresent invention, because only a limited load dump has occurred.

The other action, namely allowing temporary operation of the diodes inthe Zener breakdown even during normal generator operation, occurs, forexample, when a low diode temperature is combined with a nominal Zenervoltage of a diode at the lower edge of the Zener voltage tolerancerange, and a generator output voltage is present at the upper edge ofthe permissible vehicle electrical system operating voltage range.

According to the exemplary embodiment of the present invention,therefore, a tolerance range is provided for the Zener voltage of Zenerdiodes 2–7, the tolerance range being identified in FIG. 3 by referencenumber 15. Reference number 8 identifies a lower limit of the Zenervoltage of tolerance range 15, while reference number 9 designates anupper limit of the Zener voltage of tolerance range 15. FIG. 3 alsoshows a tolerance range 16, which provides the values at DC outputs B+,B− of rectifier 10 and may be applied to DC system 100. In this case, anupper DC voltage 12 of DC voltage range 16 and a lower DC voltage 11 ofDC voltage range 12 are provided, upper DC voltage 12 representing themaximum voltage permitted in the vehicle electrical system during a loaddump, while lower DC voltage 11 is the maximum voltage occurring duringnormal operation of the generator. Upper limit 9 of the Zener voltagemay exceed preset upper DC voltage 12 when a load dump exceeds a certainmagnitude, for example in the case of a “100% load dump” in the event ofinterruption of connection 90.

By way of example, FIG. 3 also shows a voltage scale, lower limit 8 ofthe Zener voltage being set to a voltage of 42 V, preset lower DCvoltage 11 being set to a voltage of 48 V, preset upper DC voltage 12being set to a voltage of 58 V and upper limit 9 of the Zener voltagebeing set to a voltage of 58 V or slightly higher. However, this shouldbe viewed only as an exemplary embodiment.

According to the exemplary embodiment of the present invention, themanufacturing tolerance of the Zener voltage may remain largelyunchanged compared to Zener diodes used in 14 V vehicle electricalsystems. The Zener voltage rises only slightly due to the load dumpreverse current and the diode overheating, compared to the Zener diodeused in the 14 V system. The maximum Zener voltage in realistic loaddumps relevant for vehicle electrical system components 40, 50, 60 doesnot exceed preset upper DC voltage 12. The lower tolerance ranges ofZener voltage range 15 overlap with the upper ranges of the permissibleelectrical system voltage during normal operation. In this case, theZener diode is temporarily operated in the Zener breakdown. The resultis an additional power loss in the diode. This causes the diode to heatup, while the Zener voltage continues to rise as a result of thepositive temperature coefficient of the Zener voltage and current loadin the diode, thereby also increasing the vehicle electrical systemvoltage to the maximum value required by preset lower DC voltage 11.This only slightly impairs the efficiency of generator 20 and does notlimit the long-term reliability of the diode, despite the additionalthermal load, due to its sturdy configuration.

1. A generator arrangement comprising: a generator; a diode systemincluding: Zener diodes, wherein: the diode system includes AC voltageterminals connected to corresponding AC voltage terminals of thegenerator and DC voltage terminals connected to a DC system; the Zenerdiodes are operable in a forward direction to rectify an AC voltagepresent at the AC voltage terminals of the diode system, a lower limitbeing provided for a Zener voltage of the Zener diodes; the diode systemis operable to generate a preset lower DC voltage; the lower limit ofthe Zener voltage is provided so as to be lower than the preset lower DCvoltage; a portion of the Zener diodes are operated in a Zener mode,insofar as a DC voltage that largely exceeds the lower limit is presentat the DC voltage terminals; the lower limit of the Zener voltage is setto approximately 42 V; and the preset lower DC voltage is set toapproximately 48 V.
 2. The generator arrangement of claim 1, wherein apreset upper DC voltage is a maximum applied to the DC system.
 3. Thegenerator arrangement of claim 2, wherein the preset upper DC voltage atthe generator output is exceeded when the connection between thegenerator output and the rest of the DC system is interrupted.
 4. Agenerator arrangement comprising: a generator; a diode system including:Zener diodes, wherein: the diode system includes AC voltage terminalsconnected to corresponding AC voltage terminals of the generator and DCvoltage terminals connected to a DC system; the Zener diodes areoperable in a forward direction to rectify an AC voltage present at theAC voltage terminals of the diode system, at least part of the Zenerdiodes being operated in a Zener mode for a sudden power drop, an upperlimit being provided for the Zener voltage of the Zener diodes; thediode system is provided so as to generate a preset upper DC voltage;the preset upper DC voltage is set to approximately 58 V; and the upperlimit of the Zener voltage exceeds the preset upper DC voltage if thesudden power drop exceeds a preset magnitude.
 5. The generatorarrangement of claim 4, wherein a preset upper DC voltage is a maximumapplied to the DC system.
 6. The generator arrangement of claim 5,wherein the preset upper DC voltage at the generator output is exceededwhen the connection between the generator output and the rest of the DCsystem is interrupted.